Add StreamSubscription.asFuture.

tools/dom/src/EventStreamProvider.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html;
 
 /**
  * Adapter for exposing DOM events as Dart streams.
  */
 class _EventStream<T extends Event> extends Stream<T> {
@@ skipping 79 matching lines @@
     if (_onData != null && !_paused) {
       _target.$dom_addEventListener(_eventType, _onData, _useCapture);
     }
   }
 
   void _unlisten() {
     if (_onData != null) {
       _target.$dom_removeEventListener(_eventType, _onData, _useCapture);
     }
   }
+
+  Future asFuture([var futureValue]) {
+    // We just need a future that will never succeed or fail.
+    Completer completer = new Completer();
+    return completer.future;
+  }
 }
 
 
 /**
  * A factory to expose DOM events as Streams.
  */
 class EventStreamProvider<T extends Event> {
   final String _eventType;
 
   const EventStreamProvider(this._eventType);
@@ skipping 40 matching lines @@
   const _CustomEventStreamProvider(this._eventTypeGetter);
 
   Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
     return new _EventStream(e, _eventTypeGetter(e), useCapture);
   }
 
   String getEventType(EventTarget target) {
     return _eventTypeGetter(target);
   }
 }